1. Field of the Invention
The present invention relates to a exchangeable process cartridge used for forming an image, and an image forming apparatus using this process cartridge, and, more particularly, to a process cartridge for transferring a toner image formed on a latent image carrier onto a sheet, and an image forming apparatus using this process cartridge.
2. Description of the Related Art
Image forming apparatuses, such as a copying machine, a printer and a facsimile, employ a latent image forming type recording apparatus like an electrophotographing apparatus, due to a recent demand for image recording on normal sheets of paper. According to this image forming principle, after a photosensitive drum as a latent image carrier is precharged, the photosensitive drum is exposed to a light image to have an electrostatic latent image formed thereon. This electrostatic latent image is developed by a developing unit so that a toner image is formed on the photosensitive drum. This toner image is then transferred onto a sheet of paper.
The image forming process unit such as a photosensitive drum should be exchanged with a new one since the photosensitive drum has a limited service life and the developer in the developing unit will eventually runs out. Particularly, since this image forming process unit needs to be designed smaller when installed in a compact image forming apparatus, the exchanging cycle becomes short. To facilitate the exchanging operation, the photosensitive drum, the developing unit, etc. are installed in a single cartridge so that they can be handled cartridge by cartridge. It is desirable that even when this process cartridge is exchanged, a toner image can stably be acquired.
FIGS. 1A and 1B are explanatory diagrams of prior art. As shown in FIG. 1A, the image forming apparatus has a process cartridge 160 provided above a sheet cassette 170 for retaining sheets of paper, and a stacker 173 disposed above this process cartridge 160. A U-shaped feeding path 171, which extends from the sheet cassette 170 and reaches the stacker 173 through the process cartridge 160, is formed so that sheets are conveyed along this path 171. This arrangement can prevent the sheet cassette 170 from protruding from the apparatus, thus making it possible to design the apparatus compact. Further, the sheet feeding path 171 can be made short, allowing the apparatus to become more compact. As the feeding path 171 runs vertically, it is easy to remove a jammed sheet.
The process cartridge 160, which is provided in the thus constituted image forming apparatus, has a photosensitive drum 161, a precharger for precharging the photosensitive drum 161, a developing unit for developing an electrostatic latent image on the photosensitive drum 161 with a toner, and a cleaner for removing and collecting the residue toner on the photosensitive drum 161. This process cartridge 160 is exchangeable cartridge by cartridge. Provided at the rear portion of the process cartridge 160 is an optical unit 163 for exposing the photosensitive drum 161 to a laser beam to form an electrostatic latent image thereon. A transfer roller 162 is disposed opposite to the photosensitive drum 161, and discharge rollers 172 are provided on the discharge side of the U-shaped feeding path 171.
A rotatable front cover 180 is provided at the front of the image forming apparatus. As shown in FIG. 1B, this front cover 180 is opened to allow a user to remove a jammed sheet. With the front cover 180 open, the process cartridge 160 can be pulled out so that it can be exchanged with a new process cartridge 160. This method is advantageous because the sheet cassette 170 can be loaded or unloaded from the front side of the apparatus (on the right-hand side of the drawing), and the removal of a jammed sheet and the exchanging of the process cartridge 160 become possible when the front cover 180 is opened.
With this structure, it is effective to use a transfer roller 162 as transfer means for transferring a toner image on the photosensitive drum 161 onto a sheet of paper. The use of a transfer charger is not preferable because the discharging action of the charger deteriorates the photosensitive drum and produces ozone that is harmful to human bodies. In contrast, the transfer roller 162 can transfer the toner image on the photosensitive drum onto a sheet without producing ozone harmful to human bodies and is effective to provide environmental protection.
According to known art, this transfer roller 162 is provided on the front cover 180. The transfer roller 162 performs the image transfer due to the electrostatic force and pressure while abutting on the photosensitive drum 161. The image transfer will not be performed properly unless the transfer roller 162 is accurately positioned opposite to the photosensitive drum 160 (to keep the parallelism and positioning of the shaft). For instance, the improper image transfer may result in improper printing or may cause paper jamming. According to the known art, as the accuracy of the opposing arrangement of the transfer roller 162 depends on the precision of the size of the front cover 180, it is difficult to accurately keep this opposing arrangement. In other words, if the precision of the size of the front cover 180 is improved, the opposing positional relationship between the photosensitive drum 161 and transfer roller 162 can accurately be set. This however requires a severe positional precision of the elongated front cover 180, thus resulting in an increased cost. Further, the opening face (exposed face) of the photosensitive drum 161 in the process cartridge 160 is large. Accordingly, the photosensitive drum 161 can easily be touched by hands or the like, so that the photosensitive drum is likely to be stained with oil or the like or damaged with a foreign matter, thus causing improper printing.